1. Field of the Invention
The present invention generally relates to a speaker, and more particularly, to a micro-speaker and a manufacturing method thereof.
2. Description of Related Art
A speaker produces sound by generating electrical signals and stimulating a diaphragm with the electrical signals. Speakers can be applied to various electronic products, such as cell phones, notebook computers, personal digital assistants (PDAs), digital cameras, and flat-panel TVs. Presently, the designs of different electronic products are all going towards lightness, slimness, shortness, and smallness, and high versatility. Accordingly, speakers should also be developed and manufactured through more advanced techniques in order to increase the market competitiveness thereof.
Speakers can be categorized into moving-coil speakers, piezoelectric speakers, and electrostatic speakers according to the operation principles thereof. The moving-coil speaker is currently the most broadly used and mature speaker. However, it is difficult to reduce the size of a moving-coil speaker due to the structure thereof.
According to the operation principle of the conventional electrostatic speaker, a conductive diaphragm is held between two fixed electrodes to form a capacitor. By supplying a direct current (DC) bias to the diaphragm and an alternating current (AC) voltage to the two fixed electrodes, an electrostatic force is produced by the electric fields, and the conductive diaphragm is vibrated by the electrostatic force to produce sound. However, the bias supplied to the conventional electrostatic speaker should be up to hundreds or even thousands voltages. Accordingly, an amplifier of high cost and bulky size has to be connected externally. As a result, the conventional electrostatic speaker cannot be broadly applied to different electronic products.
A piezoelectric speaker adopts the piezoelectric effect of a piezoelectric material. When an electric field is supplied to the piezoelectric material, deformation of the piezoelectric material will drive the diaphragm to produce sound. However, even though the piezoelectric speaker has a small and slim size, it is still not flexible because the piezoelectric material needs to be sintered.
A laminated piezoelectric transducer and a method for manufacturing the same are disclosed in U.S. Pat. No. 7,170,822. FIGS. 1(a)˜1(c) are diagrams illustrating the structure and laminated package of a conventional laminated piezoelectric transducer 100. Referring to FIG. 1(a), an upper and a lower layer of the laminated piezoelectric transducer 100 are two metal discs 102, and the thickness of each of the metal discs 102 is 0.005 inches. A middle layer of the laminated piezoelectric transducer 100 is a piezoelectric disc 104. Foregoing three layers form a disc structure 101 such that the amplitude can be increased. Referring to FIG. 1(b), an upper gasket 106 and a lower gasket 106 of the laminated piezoelectric transducer 100 are packaged together with the disc structure 101 to form a laminated piezoelectric transducer package 105. Then, a rubber gasket 108 is disposed to form a chamber, as shown in FIG. 1(c). According to the present disclosure, the chamber is formed in the laminated piezoelectric transducer for increasing both sound pressure and sound quality and allowing the laminated piezoelectric transducer to be applied underwater. However, because only a single-sided piezoelectric ceramic is used for driving the diaphragm, insufficient sound pressure may be caused. Besides, the laminated piezoelectric transducer in the present disclosure has very limited applications due to its low flexibility.
A piezoelectric full-range loudspeaker is disclosed in U.S. Pat. No. 5,805,726. FIG. 2(a) is a cross-sectional view of a piezoelectric full-range loudspeaker 200, and FIG. 2(b) is a top view of the piezoelectric full-range loudspeaker 200. Referring to FIG. 2(a) and FIG. 2(b), the speaker is composed of two metal alloy sheets 202 and a damping sheet 204 held between the two metal alloy sheets 202, and a sound production unit composed of a piezoelectric sheet 206 is disposed outside of the metal alloy sheets 202. Bonding wires 208 are respectively disposed outwards on the metal alloy sheets 202 and the piezoelectric sheet 206. Thus, sound can be produced when currents pass through the bonding wires. According to the present disclosure, a better sound quality is obtained by adopting the damping sheet, and the speaker in the present disclosure has such advantages as small volume, high definition sound quality, low power consumption, and no electromagnetic wave interference. Accordingly, the speaker in the present disclosure can be applied to small-sized portable electronic sound production products. However, the speaker in the present disclosure requires a very complicated manufacturing process and very high cost. Besides, because a single-sided piezoelectric sheet is adopted for driving a diaphragm having a composite structure, there may be insufficient sound pressure. Additionally, the speaker in the present disclosure has very low flexibility. Accordingly, the speaker in the present disclosure has limited applications.
A piezoelectric speaker is disclosed in U.S. Pat. No. 4,439,640. FIG. 3(a) illustrates a piezoelectric speaker 300. Referring to FIG. 3(a), a piezoelectric ceramic disc 302 and a metal disc 304 are served as the vibration source. A diaphragm 306 is disposed on the piezoelectric ceramic disc 302 and the metal disc 304. A chamber 310 is formed in the middle by using two brackets 308. Accordingly, an acoustic system is formed. FIG. 3(b) illustrates an upgraded piezoelectric speaker 300A, wherein a disc diaphragm 312 and a bracket 308 are further disposed on top. FIG. 3(c) illustrates the frequency response curves of the piezoelectric speaker 300 and the upgraded piezoelectric speaker 300A. The curves C1 and C2 respectively represent the performances of the piezoelectric speaker 300 and the upgraded piezoelectric speaker 300A.
The upgraded piezoelectric speaker 300A is more stable and has better low-frequency performance than the piezoelectric speaker 300. According to the present disclosure, a piezoelectric ceramic is used as the vibration source such that the diaphragm has higher amplitude compared to general piezoelectric materials. Besides, the speaker in the present disclosure can be applied to non-flexible electronic products. However, since a single-sided piezoelectric ceramic sheet is adopted in the present disclosure for driving a diaphragm having a composite structure, the problem of insufficient sound pressure may still exist, and also due to the low flexibility thereof, the speaker in the present disclosure cannot be broadly applied to different electronic products.
A piezoelectric structure is disclosed in U.S. Pat. No. 7,166,952. FIG. 4(a) is a top view of a piezoelectric structure 400, and FIG. 4(b) is a cross-sectional view of the piezoelectric structure 400. Referring to FIG. 4(a) and FIG. 4(b), in the present disclosure, positive/negative electrodes of a piezoelectric material are fixed to the folds 410 of a plastic material to increase the amplitude. According to the present disclosure, the amplitude is increased because of the effect of the upper separated electrodes 412 and the lower continuous electrodes 414 on the folds 410. However, the piezoelectric structure in the present disclosure requires very complicated process and high cost, and insufficient sound pressure may be caused by driving the diaphragm having the folded structure with a piezoelectric bar.